Process for solution mining nahcolite

ABSTRACT

The process of solution mining sodium bicarbonate (e.g., nahcolite) from a subsurface sodium bicarbonate containing, oil shale formation with water is improved by conducting leaching operations at a selected temperature greater than 250*F and adjusting pressure to a particular preferred value for the selected leaching temperature.

United States Patent [191 Beard et al.

[451 Dec. 18, 1973 PROCESS FOR SOLUTION MINING NAI-ICOLITE Thomas N.Beard, Denver, Colo.; Peter Van Meurs, Houston, Tex.

[75] Inventors:

Shell Oil Company, Houston, Tex.

Aug. 7, 1972 Assignee:

Filed:

Appl. No.:

U.S. Cl. 299/5, 166/303 Int. Cl E2lb 43/28 Field of Search 299/4, 5;166/272,

References Cited UNlTED STATES PATENTS 2,388,009 Pike 299 5 x Pike et a1299/5 X Papadopoulos et a1 299/5 Primary ExaminerErnest R; PurserAttorney-Theodore E. Bieber [57] ABSTRACT The process of solution miningsodium bicarbonate (e.g., nahcolite) from a subsurface sodiumbicarbonate containing, oil shale formation with water is im proved byconducting leaching operations at a selected temperature greater than250F and adjusting pressure to a particular preferred value for theselected leaching temperature.

6 Claims, 4 Drawing Figures I i-viii 7 I 1 EU um; I 8 I975 sum in; 3

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PROCESS FOR SOLUTION MINING NAHCOLITE BACKGROUND OF THE INVENTION Fieldof the Invention This invention relates to the field of producingminerals from subsurface formations; and more particularly, to a processfor solution mining nahcolite from subsurface oil shale formationsDescription of the Prior Art The recovery of water-soluble minerals fromsubsurface deposits by solution mining with aqueous fluids is wellknown. In such a process, aqueous fluid is flowed downa well intocontact with a subsurface deposit. The solution dissolves some of thesoluble mineral. The mineral-containing solvent is then flowed to thesurface where it is treated to remove the dissolved mineral, e.g., byevaporation.

The solubility of most commercially interesting water soluble mineralsincreases with increasing temperature. Therefore, aqueoussolution-mining fluid is often heated to increase its mineral carryingcapacity before it is injected into a subsurface mineral deposit. Forexample, U.S. Pat. No. 1,649,385 issued Nov. 15, 1927, to H. Blumenberg,Jr. teaches a method of solution-mining crystallized boron compounds byusing a mixture of hot air and steam.

In the western United States, there are large subsurface oil shaleformations which contain substantial amounts of water-soluble,heat-sensitive bicarbonate minerals such as trona and nahcolite. Theseminerals are present both in inter-bedded substantially pure solublemineral layers and as dispersed nodules in certain layers whichpredominently contain oil shale.

It is known that theseheat-sensitive, water-soluble minerals can besolution-mined with hot aqueous solutions. *See, for example, U.S. Pat.3,050,290, issued Aug. 21, 1962, to N. A. Caldwell et al. A co-pendingcommonly assigned application of T. N. Beard, Ser.

No. 75,009, filed Sept. 24, 1970, teaches a method of producing oil fromsuch mineral-containing oil-shale formations which includespermeabilization of the formation by dissolution of mineral with hotaqueous solution.

SUMMARY OF THE INVENTION We have now found that the process of removingheat-sensitive, water-soluble bicarbonate minerals from subsurface oilshale deposits by solution-mining with hot aqueous solutions is improvedby injecting steam into the formation at a selected temperature greaterthan 250F, and advantageously, greater than 300F, to leach water-solublemineral from the formation; maintaining the temperature of fluid in theleached zone greater than 250F; and adjusting pressure in the leachedzone to a particular optimum pressure for the selected temperature.

The optimum pressure is that pressure at which the sodiummineral-carrying capacity of the aqueous leaching fluid is at a maximum.At pressures below the optimum, excessive conversion of bicarbonatematerial to carbonate with attendant precipitation of carbonate leads'toa reduced mineral-carrying capacity. At higher pressures than theoptimum, conversion of bicarbonate material to carbonate is inhibitedand the mineralcarrying capacity of the leaching fluid is therebyreduced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a graphical representationof cavity growth rate versus cavity temperature for a nahcolite leachingoperation conducted in a nahcolite-containing oil shale formation.

FIG. 2 is a graph of sodium content expressed as equivalent pounds ofnahcolite per pound of water for a sodium carbonate saturated, sodiumbicarbonatewater system as a function of temperature.

FIG. 3 is a schematic view, partly in cross section, of asolution-mining well equipped for the practice of this invention. FIG. 4is a schematic view, partly in cross-section, of another well system foruse in the practice of this invention.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to FIG. 3, we see asubsurface oil shale formation 10 containing strata 11 of substantiallypure nahcolite (NaHCO and strata 12 which are predominantly oil shalebut which contain a substantial amount of nahcolite, e.g. 20 to 40percent nahcolite dispersed in discreet nodules.

A solution-mining well 13 extends into the oil shale formation 10 fromthe earth surface. The well 13 has been completed in a conventionalmanner with casing 14 sealed in place with cement 15. A solution-miningfluid injection tubing string 16 and a solution-mining fluid productiontubing string 17 are extended into the well 13. The lower end of theinjection tubing 16 is preferably positioned adjacent the top of a zone9 of the oil shale formation 10 to be solution-mined. The lower end ofthe production tubingstring is preferably positioned near the bottom ofthe zone 9.

Pack-off means such as packer 18 may be positioned in the casing 14above the lower end of the tubing string 16. Production tubing string 17is provided with suitable means for lifting solution-mining fluid to thesurface. For example, pumping apparatus may be positioned adjacent thebottom of production string 17 or the production string 17 may beequipped for gas lift as shown in FIG. 3. In the embodiment illustrated,a pressure actuated gas lift valve 19 is operatively connected toproduction tubing 17 at a point above packer 18. A conduit 20 forinjection gas is connected to the casing 14 at the surface. To liftfluid in the tubing 17, gas is injected through conduit 20 into casing14. When the pressure of this gas exceeds a certain threshold value,valve 19 opens and admits gas into the interior of tubing 17. This gaslightens the column of fluid in tubing 17 thereby reducing the pressurenecessary to cause fluid to flow from the bottom of tubing 17 to theearth surface.

To solution mine nahcolite from formation 10, hot aqueoussolution-mining fluid, preferably low quality steam, is injected downtubing 16. This fluid contacts water-soluble minerals in the formation10 and dissolves them thereby forming a leached zone and, eventually, acavity 21. The cavity 21 may be at least partially filled withfragmented particles of oil shale and nahcolite 22.

We have found that in leaching formations similar to that shown in FIG.1 with steam, the cavity growth rate varies logarithmically with thecavity temperature as shown in FIG. 1 and that cavity growth rate isonly slightly dependent upon the rate of fluid injection. It is believedthat this increase in cavity growth rate with temperature is at least inpart due to more rapid thermal fracturing at higher temperatures of oilshale surrounding discreet nahcolite nodules. Such fracturing allowsinjected aqueous fluid to reach the nahcolite nodule and leach it fromthe formation leaving an exposed oil shale face which is in turnthermally fractured opening up communication to yet another nodule.

As can be seen in FIG. 1, for temperatures below 250F, growth rate ofcavity radius is quite low, less than 0.08 feet per day; whereas at300F, growth rate is almost doubled to 0.15 feet per day. Thus, formaximum mineral removal, cavity temperature should be maintained above250F and preferably above 300F.

We have also found that in solution-mining nahcolite from an oil shaleformation with aqueous fluid, the rate of mineral recovery can bemaximized by selecting an operating temperature for maximum desiredcavity growth rate as by reference to FIG. 1, and then during operationadjusting cavity pressure to a pressure at which the sodium carryingcapacity of the aqueous leaching fluid is a maximum for the selectedcavity temperature. This pressure is less than that required tobydraulically fracture the formation and is greater than the pressure atwhich nahcolite decomposition to sodium carbonite, carbon dioxide andwater is maximized.

Operating in this manner can significantly reduce the energy requirementfor carrying out the process since heat can be carried to the formationby relatively low pressure steam. Additionally, water requirements arereduced because the total amount of sodium mineral removed from thecavity 21 by a given volume of leaching luid is maximized.

The particular selected leaching temperature will vary from operation tooperation depending upon economic conditions and the desired cavitygrowth rate for each particular case. Operating pressure for aparticular selected temperature is determined from pressure,temperature, saturation relationships such as those given in FIG. 2.That figure shows total sodium concentration in pounds of nahcolite perpound of water for a sodium carbonate saturated, sodium carbonate/-sodium bicarbonatewater system. The graph reflects the amount ofnahcolite removed from a nabcolite formation which is present in thesolution even though the actual composition of the solution includesboth sodium bicarbonate and sodium carbonate generated by nahcolitedecomposition. Best results are obtained by operating at the pressurefor which the isobar intersects the upper dashed curve at the selectedoperating temperature. Good results are obtained at pressures varying asmuch as percent above or below this pressure.

Looking at FIG. 2 for a temperature of 400, it can be seen that at thattemperature and about 200 psi only sodium bicarbonate is present in thesolution (as given by the lower dotted line of the Figure) and that thetotal amount of equivalent nahcolite dissolved is around 0.55 pounds perpound of water. However as pressure is increased, the amount of sodiumbicarbonate in the system increases until at about 1,000 psi, the totalsodium content is equivalent to about 1.25 pounds per pound of watereven though sodium carbonate saturation remains the same. Furtherpressure increase to a pressure for which the extention of an isobarwould intersect the 400F isotherm above the upper dotted line results inthe precipitation of sodium bicarbonate and an effective reduction inthe equivalent nahcolite saturation of the system. Thus at 400F,leaching operations can be maximized if pressure in the cavity 21 ismaintained at about 1,000 psi. To maintain this pressure, it isnecessary to artiflcally lift fluid from the cavity 21 if the fluid headof solution-mining fluid in production tubing 17 is greater than 1,800psi. Therefore, the well 13 is provided with a gas lift system asheretofore described.

FIG. 4 shows a well 22 extending into the formation 10 that is completedin a manner particularly advantageous for the practice of thisinvention. The well 22 is completed with casing 23 which extends intothe nahcolite-containing formation 10. The casing 23 is cemented inplace with cement 24 and perforated adjacent formation 10 withperforations 25 to open the interior of the casing into communicationwith the formation 10.

A liquid production tubing string 26 and a gas production tubing string27 extend into the well from the surface. The liquid production tubingstring 26 preferably terminates at the point adjacent the bottom of theinterval of the formation 10 to be treated whereas the gas productiontubing string 27 terminates at a point above the lower end of the liquidproduction tubing 26 but below the perforations 25. The interior of thecasing is preferably sealed to fluid flow by pack-off means such aspacker 28 at a point above the terminal ends of the two tubing strings26 and 27 and below the perforations 25.

The liquid production tubing string is provided with means for liftingliquid from the formation 10 to the surface. This may be a down-holepump or gas lift means (as illustrated in FIG. 4) in which a gasinjection string extends into the well 22 and is connected incommunication with production tubing 26 at a point near the lower end ofthat tubing. The particular point of intersection will be determined bythe fluid head desired to be maintained in liquid production string 26.

In operation, hot aqueous fluid having a temperature greater than 250and preferably greater than 300F is injected into casing 23 throughconduit 30 and then down the casing until it passes through perforations25 into the formation 10. This fluid leaches nahcolite from theformation creating a cavity 31 which may be filled with fragmentedparticles of oil shale and nahcolite 32. The aqueous fluidadvantageously contains high proportion of steam which upon contactingthe formation 10 condenses to form a liquid phase capable of carryingdissolved mineral in solution. Simultaneously with the injection ofsteam down the casing 22, liquid is produced from the lower part of thecavern 31 through production tubing string 26 and gas is produced fromthe cavern 31 through gas production tubing string 27. The productionrate of these fluids is preferably adjusted to maintain the pressure inthe cavern 31 at a particular preferred value for the selectedtemperature operation. The removal of gas through the tubing 26 drawsboth steam and CO from the cavern 31. This results in a reduction of thepartial pressure of CO in the cavern and further promotes thedecomposition of nahcolite (NaHCO to sodium carbonate, CO and water(e.g., 2 NaHCO, Na CO3 C0, H O).

Both FIGS. 3 and 4 illustrates single well systems for the practice ofthis invention. However, it should be understood that two or more wellsmay at any one time be in communication with any particular cavern 21 or31 or other permeabilized zone. In such a case, aqueous fluid may beinjected into the formation through one well and produced from theformation through a separate well.

Both FIGS. 3 and 4 illustrate the process after a cavity 21 or 31 hasbeen formed. It should be understood that in many cases, initialtreatment will be confined to a substantially cylindrical wellbore andthat the cavern is formed only after a period of leaching has expandedthe wellbore radically.

We claim as our invention:

1. In a method for solution-mining heat sensitive water-soluble sodiumbicarbonate minerals from a subsurface bicarbonate mineral containingoil-shale formation of the type wherein a hot aqueous fluid is injectedinto the formation to leach bicarbonate mineral therefrom, theimprovement comprising:

injecting steam into the formation at a temperature greater than 250F toleach water-soluble mineral from the formation and thereby create aleached zone; maintaining the temperature of fluid in the leached zoneat a temperature greater than 250F; and

adjusting pressure in the leached zone to an optimum pressure at whichthe sodium mineral carrying capacity of water at the selectedtemperature is a maximum.

2. The method of claim 1 further comprising producing liquid containingdissolved sodium bicarbonate from a liquid layer adjacent the bottom ofthe leached zone through a production tubing string using artificiallift means to lift the liquid to the surface.

3. The method of claim 2 further comprising withdrawing gas containingCO from a gas layer adjacent the top of the leached zone.

4. A method for solution-mining nahcolite from a subsurface oil-shaleformation comprising the steps of:

traversing a nahcolite-containing zone of the oilshale formation with awell;

extending a production string of tubing into the well to a pointadjacent the bottom of the nahcolitecontaining zone;

extending an injection tubing string into the well to a point adjacentthe top of the nahcolite-containing zone; injecting steam into contactwith the nahcolite containing zone through the injection tubing at atemperature such that upon contacting the formation at least some of thesteam condenses to liquid which liquid flows to the bottom of thenahcolite zone leaching nahcolite therefrom; producingnahcolite-containing aqueous liquid from the nahcolite zone through theproduction tubing;

controlling the rate and temperature of steam injection to maintain aselected temperature of aqueous liquid adjacent the bottom of thenahcolite zone;

adjusting the pressure in the aqueous liquid adjacent the bottom of thenahcolite zone to an operating pressure substantially equal to thatpressure at which the amount of sodium mineral the aqueous liquid cancarry at the selected temperature is a maximum; and

maintaining the pressure in the aqueous liquid adjacent the bottom ofthe nahcolite zone substantially constant at the operating pressure.

5. The method of claim 4 wherein the operating pressure is a pressureless than that required to hydraulically fracture the formation.

6. The method of claim 5 wherein the operating pressure is greater thanthe pressure at which the rate of nahcolite decomposition is a maximumat the selected temperature.

2. The method of claim 1 further comprising producing liquid containingdissolved sodium bicarbonate from a liquid layer adjacent the bottom ofthe leached zone through a production tubing string using artificiallift means to Lift the liquid to the surface.
 3. The method of claim 2further comprising withdrawing gas containing CO2 from a gas layeradjacent the top of the leached zone.
 4. A method for solution-miningnahcolite from a subsurface oil-shale formation comprising the steps of:traversing a nahcolite-containing zone of the oil-shale formation with awell; extending a production string of tubing into the well to a pointadjacent the bottom of the nahcolite-containing zone; extending aninjection tubing string into the well to a point adjacent the top of thenahcolite-containing zone; injecting steam into contact with thenahcolite containing zone through the injection tubing at a temperaturesuch that upon contacting the formation at least some of the steamcondenses to liquid which liquid flows to the bottom of the nahcolitezone leaching nahcolite therefrom; producing nahcolite-containingaqueous liquid from the nahcolite zone through the production tubing;controlling the rate and temperature of steam injection to maintain aselected temperature of aqueous liquid adjacent the bottom of thenahcolite zone; adjusting the pressure in the aqueous liquid adjacentthe bottom of the nahcolite zone to an operating pressure substantiallyequal to that pressure at which the amount of sodium mineral the aqueousliquid can carry at the selected temperature is a maximum; andmaintaining the pressure in the aqueous liquid adjacent the bottom ofthe nahcolite zone substantially constant at the operating pressure. 5.The method of claim 4 wherein the operating pressure is a pressure lessthan that required to hydraulically fracture the formation.
 6. Themethod of claim 5 wherein the operating pressure is greater than thepressure at which the rate of nahcolite decomposition is a maximum atthe selected temperature.